Use markdown in long_description_content_type

Author: Doru Irimescu

setup.cfg

@@ -8,7 +8,7 @@ description = Wait and retry calling an exception throwing method.
 author = Doru Irimescu
 license = MIT
 long_description = file: README.md
-long_description_content_type = text/x-rst; charset=UTF-8
+long_description_content_type = text/markdown; charset=UTF-8
 url = https://github.com/doruirimescu/exception-with-retry/
 
 # Change if running only on Windows, Mac or Linux (comma-separated)